Component durability is only a small part of the start-stop puzzle. Suppliers and automakers alike look at start-stop as a system that's integral to the overall design of the vehicle.
"The key is to make it unnoticeable to the user," Patton says. "The driver shouldn't even know the start-stop system is there."
Making that happen isn't easy. The first step is in the vehicle's engine control module (ECM). Engineers say the vehicle needs to "know" whether start-stop activation is imminent. To do that, it checks sensors to determine if the car is stopped and the brake pedal is depressed.
That's only the beginning. Even when a microcontroller determines that the vehicle is stopped, it still has to go through a long list of "enablers" and "disablers." Is the seat belt fastened? Is the engine fully warmed up? Does the battery have adequate charge to restart the car? In all, the vehicle's checklist is likely to have about 30 items. Once it is satisfied, the vehicle will shut down the engine and keep it that way as long as the driver keeps a foot on the brake pedal.
When the foot comes off, everything changes again. The starter cranks, a pinion is thrust back into the drivetrain's ring gear, and the engine breathes again. That process -- from brake pedal to accelerator to motion -- is expected to take about half a second.
And therein lies one of the start-stop's great challenges. To make the process invisible to the driver, automotive vendors need to supply a system that comes back to life quickly and quietly. Drivers, it seems, don't want to have to wait five seconds when a traffic light turns green.
The key to faster starts lies in the control strategy. In the past, the driver activated the ignition switch, causing a solenoid to engage the starter's pinion, which inserted itself into the drivetrain's ring gear, and the engine turned over. Now engineers want to be ready to engage the pinion earlier in the process. A more robust crankshaft sensor can continually measure the position of the crankshaft, thereby knowing piston position. As a result, the controller "knows" which cylinder needs to have fuel injected into it.